The present invention relates to a method and a device according to the preambles of the appended independent method and device claims, which aim at reducing the influence of exterior disturbing magnetic fields upon the measurement of quantities through inductive measurement technique.
The invention may be used for finding metal objects, as nails and other metal objects in logs or metal ribbons in paper handling. The invention may also be used for inductive measurement of quantities of metal products, such as thickness of plate ribbons or the diameter of rods or other geometric measure of metal products. The invention may also be useful for measuring electric and magnetic properties of metal products.
To detect the presence of metal objects by means of inductive technique and to measure the size of metal objects by means of inductive technique has to be considered to be a technique known for a long time. A magnetic field varying with the time is then normally generated by feeding a coil with a current varying regularly with the time. The field so generated interacts with the metal object in the field region and induces currents in the object, which in their turn creates magnetic fields superposed on the field applied. The two fields, the applied one and the superposed from the object, are normally measured by a coil, a receiving coil, in which the two fields will induce a voltage which will be a primary measure of the quantity asked for. The generating coil and the receiving coil may be separate coils or the same coil and the transmitting as well as receiving coil may be one coil or an arrangement of coils.
The current varying regularly with the time feeding the generating coil is normally a sinusoidal current having a constant fre quency, such as described in U.S. Pat. No. 4,475,083, but it may also be a current having a rectangular wave shape with a constant time length, such as described in U.S. Pat. No. 5,227,902, or it may be of another shape being regular with respect to the time. The time for the regular repetition of the current is called the time length of the feeding current or of the measurement or as an alternative, the frequency by which the regularity is repeated is called the frequency of the feeding current or of the measurement.
The currents induced in the object and so the superposed field from the object, which through induced voltage in a receiving coil is measured as a measure of objects, will be totally controlled by the generating field. The consequence thereof is that the induced voltage in the receiving coil will have the same regularity with respect to the time as the current generating a field emanating from the basic signal. This is often a great advantage as it is on one hand known in which period of time the induced voltage from the object is received, on the other by that fact that it is possible to use the progress of the generated current with the time so as to control the analysis of the induced voltage. The fact first mentioned makes it possible to make filters for removing undesired noises having other periods of time with a high accuracy according to known filter technique. The fact last mentioned means that the signal may be treated very exactly for detecting the voltage superposed from objects selectively from the voltage caused by the applied field in the receiving coil, by synchronous rectifying, such as described in SE 7710481-8, or by analysis of determined periods of time, such as described in U.S. Pat. No. 5,059,902.
A problem when using the above measuring and detection technique in industrial connections is the fact that big electric machines close to such a meter produce strong magnetic fields varying with the time, which often may have a length of time, with a regular time progress, which is (nearly) the same as the length of time of the current applied. The consequence of this is that the outer disturbance may not be noticeably suppressed by the technique known so far.
A way to reduce the outer disturbance in an inductive meter is to build it in in any type of shield, but this is often impossible in industrial applications, since this requires a large space often not available. Furthermore, such a meter has to have large openings for introducing and taking out material to be measured and disturbing fields may enter through these openings.
The problems with an influence of outer disturbing fields will be solved by using the frequency modulation technique known from the telecommunication technique, i.e. to continuously and in a controlled way change the length of time. However, this way to proceed means that a larger number of regular time progresses have to be treated for eliminating the disturbance, which in its turn means that the measurement often gets too slow for the need of fastness existing. As an example the industrial search for nail in logs may be mentioned, in which a possible nail may manage to pass the meter during these time progresses. Furthermore, the use of the modulation technique mentioned above means that filter constructed with a narrow band may not be used to the same extent, which produces a positive effect of the modulation.
The object of the present invention is to provide a method and a device for inductive measurement of a quantity, which makes it possible to substantially reduce the negative influence of exterior disturbance fields varying regularly with the time mentioned above having a time length of the regularity being close to the time length used in the measurement.
This object is according to the invention obtained by that the polarity of the basic signal sent to the coil is changed regularly and synchronized with the length of time for the regular repetition of the basic signal, so that the coil is brought to generate a magnetic field on the basis of the basic signal so modified, an analysis of the detection signal induced in the region of measurement is controlled in synchronisation with this change of polarity, and a polarity change of the detection signal is carried out in synchronisation with the polarity change of the basic signal before this analysis, in which polarity change means that the signal in question thereafter takes an opposite sign to the sign it would have had without such a change until a subsequent such change, after which it gets the same appearance as had none of the two last polarity changes taken place. This polarity change may either take place with the length of the time of regular repetition of the basic signal, i.e. the time length, or any exact multiple thereof and gives a regular determined pattern. A device according to the invention has a first member adapted to change the polarity of the basic signal sent to the coil in a regular way on the basis of a length of the time of a regular repetition of the basic signal or a multiple thereof, and a second member adapted to change the polarity of the detection signal synchronized with the polarity change by the first member in connection with the treatment of the detection signal, in which polarity change means that the signal in question thereafter takes an opposite sign to the sign it would have had without such a change until a subsequent such change, after which it gets the same appearance as had none of the two last polarity changes taken place, so that disturbing signals with a length of time close to the length of time of said basic signal will be eliminated if the detection signal is summarized over a measurement time period extending at least over the period of time between said regular changes of polarity of the detection signal.
By ensuring that the components of the detection signal emanating from the basic signal are influenced twice in this way, but the components emanating from the disturbance signal are only influenced once it will be possible to make the influence through said second member so that said addition will substantially give a zero signal with respect to the disturbance signals, without causing the very measuring signal, i.e. the basic signal possibly influenced in the region of measurement, to disappear by the integration of the signal, but the information about this quantity remains and may be determined with a high reliability.
According to a preferred embodiment of the invention said first and second members for signal influence are coordinated in such a way that the second member influences the detection signal to change polarity inversely to the influence by the first member upon the basic signal, so that components included in the detection signal subjected to an influence of the first as well as the second member will have substantially the same appearance as had they not been subjected to any influence at all. This constitutes a very simple and advantageous solution to the problem to ensure that the integration over the measurement time period of components of the detection signal emanating from disturbing signals will be substantially zero at the same time as components thereof emanating from the basic signal and possible influence thereupon through the measurement in the region of measurement give a well-measurable result, since in this case the components last mentioned will remain substantially uninfluenced by the first and second members. However, it is pointed out that it is within the scope of the invention to carry out different signal influence by the first and second members, as long as it is obtained by the signal influence through the first members that the very measurement signal through influence by the second members will not receive such an appearance that it will be substantially zero when carrying out said addition, but will continuously be well interpretable.
According to another preferred embodiment of the invention the member for generating the basic signal is in a first case adapted to continuously vary the time length of the basic signal, i.e. the length of the time for the regular repetition of the basic signal, and in a second case to continuously vary the frequency of the basic signal. The polarity change of the basic signal and the synchronously controlled change of the detection signal as above may be combined with the change of the time length or alternatively the frequency of the basic signal progress over time. Especially in measuring situations where disturbances are large and contain different time length components, or alternatively frequency components, in which any of these components has substantially the same time length as the basic signal, such a combination may give an efficient disturbance elimination. The time length of the basic signal may then as for frequency modulation be continuously varied around a midvalue and the polarity of the basic signal of the detection signal is at the same time turned as above.
Further advantages as well as advantageous features of the invention will appear from the following description and the other dependent claims.